1. Technical Field of the Invention
This invention relates to foundry sand compositions of the type used to form self-hardening sand cores and molds for use in metal casting. The foundry sand compositions are prepared from a foundry sand, an aqueous solution of sodium silicate and a polyester polycarbonate as hereinafter defined.
In accordance with a preferred embodiment of the present invention, a foundry sand is mixed with about 2 to about 6 wt. %, based on the foundry sand, of an aqueous solution containing from about 40 to about 60 wt. % of sodium silicate, the ratio of the SiO.sub.2 /Na.sub.2 O of the sodium silicate being within the range of about 2.2 to 3 to form an initial sand mixture to which a polyester polycarbonate hardener is added in an amount of about 5 to about 15 wt. %, based on the weight of the aqueous solution of sodium silicate. The thus-prepared foundry sand compositions will normally have a working life of about 10 to about 20 minutes and will thereafter gel and harden. While the foundry sand composition is still pliable and before it has gelled, it is shaped, for example, in a core box into which a model of a core is placed to form a design so that, after the foundry sand composition gels and hardens, the hardened, formed mold may be used to cast a metal core.
2. Prior Art
The use of mixtures of sand with a binder to prepare molds for metal casting is well-known. See, for example, the McGraw-Hill "Encyclopedia of Science and Technology", 5th Edition, Vol. 8, pp. 392-396 (McGraw-Hill Book Company, New York, St. Louis & San Francisco).
Stevenson et al. U.S. Pat. No. 4,416,694 discloses foundry sand compositions made from a foundry sand, an aqueous sodium silicate binder and an alkylene carbonate which are used to form molds and/or cores in metal casting. The foundry sand in the foundry sand compositions disclosed by Stevenson et al. is reclaimed after the mold or core has served its purpose in metal casting.
Cuscurida et al. U.S. Pat. No. 4,267,120 is directed to polyester polycarbonates of the type used in the practice of the present invention and to methods by which they can be prepared. Cuscurida et al. teach that the polyester polycarbonates can be used in making polymer foams including polyurethane polymers and polyisocyanurate polymers.
Gaul et al. U.S. Pat. No. 4,359,507 is directed to an adhesive binder composition for the preparation of lignocellulosic composite molded articles made from organic polyisocyanates and a liquid mixture of either ethylene carbonate or propylene carbonate with lignin and other appropriate lignocellulosic materials.
Cannarsa et al. U.S. Pat. No. 4,773,466 is directed to the evaporative casting of molten metals using copolymer polycarbonates prepared from cyclohexene oxide, cyclopentene oxide, heptene oxide or isobutylene oxide and carbon monoxide.
Cuscurida et al. U.S. Pat. No. 4,488,982 is directed to improved surfactants in functional fluids prepared by reacting a monofunctional initiator with an alkylene carbonate or with an alkylene oxide and carbon dioxide to form a polyether polycarbonate material.
A trade brochure entitled "Foundry Practice 213", dated August, 1986, and published by Foseco International, Ltd., Birmingham, England, describes a method for preparing molds from sand and a binder composition and for reclaiming the foundry sand used in making the mold. Binders, such as mixtures of an aqueous solution of sodium silicate with an alkylene carbonate, as disclosed in Stevenson et al. U.S. Pat. No. 4,416,694 may be mixed with the foundry sand and used in preparing the molds.